Overlap-welded member, automobile part, method of welding overlapped portion, and method of manufacturing overlap-welded member

ABSTRACT

The present invention provides an overlap-welded member in which an overlapped portion including plural steel sheet members is joined at a spot-welded portion, in which at least one of the plural steel sheet members contains martensite, and the spot-welded portion includes: a nugget formed through spot welding; a heat-affected zone formed in the vicinity of the nugget; the softest zone having the lowest Vickers hardness in the heat-affected zone; and a tempered area formed between a central portion of the nugget and the softest zone and made out of tempered martensite having Vickers hardness of not more than 120% in the case where Vickers hardness of the softest zone is 100%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of copending application Ser. No.14/418,403, filed on Jan. 29, 2015, which is a national phase of PCTInternational Application No. PCT/JP2013/071841 filed on Aug. 12, 2013,which claims the benefit under 35 U.S.C. § 119(a) to Patent ApplicationNo. 2012-178691, filed in Japan on Aug. 10, 2012, all of which arehereby expressly incorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to an overlap-welded member obtained byjoining an overlapped portion including plural steel sheet members at aspot-welded portion, an automobile part including the overlap-weldedmember, a method of welding the overlapped portion, and a method ofmanufacturing the overlap-welded member.

The present application claims priority based on Japanese PatentApplication No. 2012-178691 filed in Japan on Aug. 10, 2012, thecontents of which are incorporated herein by reference.

BACKGROUND ART

In recent years, in the automobile field, high-tensile steel sheets havebeen increasingly used in order to reduce the weight of a vehicle andimprove safety against collision.

Furthermore, the level of strength of high-tensile steel sheets hasincreased year by year, and for example, hot-stamped members havingtensile strength of 1500 MPa or higher have been practically used. Thehot-stamped member as used herein is a member obtained by applying pressworking in a state where a steel sheet is heated to approximately 900°C. to be softened, and at the same time, quenching and strengthening thesteel sheet using a cooling effect (contact cooling) due to contact witha die, thereby achieving the tensile strength of a 1500 MPa class asdescribed above and favorable dimensional accuracy.

Furthermore, for example, in the case of assembling a vehicle body, aresistance spot welding is frequently used in which two or more steelsheet members formed by steel sheets are overlapped, and energization isapplied while pressure is being applied with electrodes.

With this resistance spot welding, a melted and solidified portionhaving an ellipse shape, in other words, a nugget is formed in theoverlapped portion through energization and heating, whereby it ispossible to join the plural steel sheet members.

For example, FIG. 1 is a diagram schematically illustrating distributionof hardness in a spot-welded portion 10 in the case where conventionalenergization conditions are applied to two transformation-inducedplasticity (TRIP) members S11 and S12.

More specifically, (a) in FIG. 1 is a sectional view schematicallyillustrating the vicinity of the spot-welded portion 10 in which thevertical direction on the paper is set to the thickness direction (inother words, a direction in which pressure is applied with theelectrodes) of the TRIP members S11 and S12. Note that, in the followingdescriptions in the specification of the present application, a diagramillustrating a cross-section of two overlapped members when viewed in asimilar manner to that in (a) in FIG. 1 is also referred to as a“sectional view illustrating a/the spot-welded portion.”

Furthermore, (b) in FIG. 1 is a graph schematically illustratingdistribution of Vickers hardness so as to correspond to (a) in FIG. 1.

A molten metal generated through resistance spot welding is cooled at ahigh cooling rate, and hence, martensite is more likely to form in anugget 12. As a result, the nugget 12 has a structure harder than a basemetal portion. Note that, in the case where the strength of the basemetal is high, the carbon equivalent is generally high, so that Vickershardness of the nugget is high.

As illustrated in FIG. 1, the spot-welded portion 10 includes the nugget12 and a HAZ 14. The HAZ 14 includes a HAZ hardened portion 14H locatedclose to the nugget 12, and a HAZ softening zone 14T formed in thevicinity of the HAZ hardened portion 14H. Furthermore, the softest zone14L in HAZ exists at an inner peripheral edge of the HAZ softening zone14T.

The quality of the spot-welded portion is often evaluated on the basisof a tensile shear strength and a cross-tension strength (the strengthof joint in a peel direction), and it is known that the tensile shearstrength increases with an increase in the strength of the base metal.

However, in the case where the base metal has a tensile strength higherthan a 780 MPa class, the peel strength, typified by the cross-tensionstrength, tends to decrease with an increase in strength of the basemetal.

Below, a cross-tension test based on JIS Z3137 (1999), which is designedfor measuring the cross-tension strength, will be schematicallydescribed with reference to FIG. 2A.

As illustrated in FIG. 2A, in the cross-tension test, two test piecesS21 and S22 formed by steel sheets are orthogonally arranged, and arejoined by forming the spot-welded portion 10 including the nugget 12through resistance spot welding.

Then, the test pieces S21 and S22 are pulled in a direction in whichthey are peeled, and the peel strength is measured until the spot-weldedportion 10 is fractured.

A fracture mode with the cross-tension test can be divided into thefollowing:

(a) interface fracture in which an interface between sheets in thenugget fractures;(b) partial plug fracture in which, as illustrated in FIG. 2B, a crackpropagates within the nugget 12 (inner side than a nugget end 12E) andthen, fracture advances in the thickness direction; and(c) plug fracture in which, as illustrated in FIG. 2C, the nugget 12does not break, and the outer peripheral portion of the nugget 12fractures in the thickness direction.

FIG. 2D is a diagram illustrating an example of a correlation between abase metal tensile strength and a cross-tension strength.

In FIG. 2D, “black dots” represent the plug fracture, and “blankcircles” represent the partial plug fracture.

As illustrated in FIG. 2D, the cross-tension strength is approximately 9kN in the case of a hot-stamped member of a 1500 MPa class (a steelsheet member obtained by hot-stamping a steel sheet for hot-stampingwhose tensile strength becomes a 1500 MPa class by being hot-stamped),and is approximately 4 kN in the case of a hot-stamping member of an1800 MPa class (a steel sheet member obtained by hot-stamping a steelsheet for hot-stamping whose tensile strength becomes an 1800 MPa classby being hot-stamped).

On the other hand, the cross-tension strength of a high-strength steelsheet of a 980 MPa class or lower falls in the range of approximately 8kN to 14 kN.

In other words, the cross-tension strength of the hot-stamped member ofa 1500 MPa class or higher is significantly lower than that of thehigh-strength steel sheet of 980 MPa class or lower.

Furthermore, as for the fracture mode through the cross-tension test,the high-strength steel sheet of a 980 MPa class or lower is fracturedmainly in relation to the plug fracture in which the outside of thenugget 12 fractures, whereas the hot-stamp member of a 1500 MPa class orthe hot-stamped member of an 1800 MPa class is fractured mainly inrelation to the partial plug fracture.

This shows that, in the case of the hot-stamped member of a 1500 MPaclass or higher, a crack is more likely to occur in the nugget becausethe toughness is small in the nugget.

As described above, in the case of the spot welding of the high-strengthsteel sheet, it is considered that the peel strength reduces mainlybecause the toughness reduces with an increase in hardness of thenugget, and thus, fracture (partial plug fracture) is more likely tooccur in the nugget.

In general, with an increase in the diameter of the nugget, the fracturemode is more likely to be the plug fracture rather than the partial plugfracture, and the strength of the spot-welded portion increases.

Thus, in order to improve the peel strength of the spot-welded portionof the high-tensile steel sheet, it is effective, for example, toincrease the diameter of the nugget.

However, in the case where the high-tensile steel sheet is subjected toresistance spot welding, spattering of molten steel called splash ismore likely to occur as compared with a case where mild steel issubjected to resistance spot welding, possibly making it difficult toincrease the diameter of the nugget.

In order to suppress the occurrence of splash, it is effective, forexample, to increase the compression force with the electrodes. However,there is a restriction resulting from equipment such as a limitation ofa welding gun in terms of stiffness.

Furthermore, it can be considered that, by increasing the number ofspots in spot welding, it is possible to reduce the load stress per spotin spot welding. However, deterioration in productivity is inevitable.

Furthermore, if the distance between spots in spot welding is reduced,electric current is diverted to the spot-welded portions that have beenalready formed, causing a problem in which nuggets cannot be formed in astable manner.

In other words, a desirable technique is one that can improve thestrength of an overlap-welded member with resistance spot weldingwithout changing the diameter of the nugget from the conventional one.

As for the technique described above, a subsequent energization methodis disclosed in which a nugget is formed with main energization, andafter the nugget is cooled, energization is performed again (see, forexample, Non-Patent Document 1).

With the subsequent energization method, as illustrated, for example, inFIG. 3, in a state where a predetermined compression force is appliedwith electrodes in resistance spot welding,

(A) a nugget is formed by applying first energization (mainenergization) under conventional normal conditions;(B) a predetermined suspension time is set to cool until martensite isformed in the vicinity of the nugget; and(C) second energization (subsequent energization) is applied, therebytempering the martensite.

With the subsequent energization method as described above, eachheat-affected zone (hereinafter, referred to as a HAZ) of the nugget andthe spot-welded portion is tempered, whereby toughness is improved.Furthermore, the HAZ is softened and is easily deformed, whereby stressin a nugget end portion area is alleviated at the time of peeling. Thus,it is considered that the peel strength can be improved.

With the resistance spot welding employing the subsequent energization,after the nugget is formed through the main energization, the moltenmetal is rapidly cooled through an Ms point to an Mf point or lower, andmartensite is formed.

The martensite thus formed becomes tempered martensite by controllingthe electric current conditions and the like used in the subsequentenergization to adjust a heat-inputted amount so as to raisetemperatures to fall in an appropriate temperature range (in otherwords, not less than approximately 550 to 600° C. and not more than anAc1 point as illustrated in FIG. 3) in which tempering is possible, andbeing cooled after the subsequent energization is completed.

FIG. 4 is a diagram schematically illustrating distribution of hardnessin a spot-welded portion 10 after the spot-welded portion 10 is formedby overlapping test pieces S31 and S32, which are dual phase (DP)members or TRIP members, under normal conditions used in theconventional resistance spot welding illustrated in FIG. 3, and applyingthe subsequent energization.

More specifically, (a) in FIG. 4 is a sectional view illustrating aspot-welded portion, and (b) in FIG. 4 is a graph schematically showingdistribution of Vickers hardness in which each position corresponds tothat in (a) in FIG. 4.

In the case where the overlapped portion is welded through resistancespot welding using the subsequent energization as illustrated in FIG. 3,the spot-welded portion 10 is first formed through main energization.

At this point in time, as illustrated in (b) in FIG. 1, the spot-weldedportion 10 includes the nugget 12 and the HAZ 14, and the HAZ 14includes a HAZ hardened portion 14H proximate to the nugget 12, and aHAZ softening zone 14T formed in the vicinity of the HAZ hardenedportion 14H. Furthermore, the softest zone 14L in HAZ exists at an innerperipheral edge of the HAZ softening zone 14T.

Then, by applying the subsequent energization to the spot-welded portion10, the nugget 12 and the HAZ hardened portion 14H are tempered asillustrated in FIG. 4, and the hardness of the nugget 12 and the HAZhardened portion 14H is decreased.

However, hard portions 14P locally remain in the HAZ hardened portion14H. Thus, at the time of peeling, the hard portions in the HAZ 14 arenot deformed, and deformation concentrates on the vicinity of the nuggetend 12E. As a result, stress concentration on the nugget end 12E is notsufficiently improved.

Furthermore, FIG. 5 is a diagram schematically illustrating changes of aHAZ 14 in a spot-welded portion 10 in the case where resistance spotwelding according to conventional normal conditions is applied to testpieces S41 and S42, which are hot-stamped member, to form thespot-welded portion 10, and the spot-welded portion 10 is subjected tosubsequent energization.

More specifically, (a) in FIG. 5 is a sectional view illustrating aspot-welded portion including the nugget 12 formed through singleenergization applied to the test pieces S41 and S42, and (b) in FIG. 5is a graph schematically showing distribution of Vickers hardness inwhich each position corresponds to that in (a) in FIG. 5.

Furthermore, (c) in FIG. 5 is a sectional view illustrating aspot-welded portion including the nugget 12 after the subsequentenergization, and (d) in FIG. 5 is a graph schematically showingdistribution of Vickers hardness in which each position corresponds tothat in (c) in FIG. 5.

It should be noted that the long dashed double-short dashed lineillustrated in (d) in FIG. 5 illustrates the distribution of Vickershardness after the main energization and before the subsequentenergization.

In the case where the subsequent energization is performed underappropriate conditions, a large area including the nugget 12 and the HAZhardened portion 14H is tempered as illustrated in (d) in FIG. 5.However, tempering cannot be sufficiently performed between the nuggetend 12E and the softest zone 14L in HAZ, and portions 14P having highVickers hardness locally remain.

In other words, an effect of improving toughness through temperingcannot be sufficiently obtained, and hence, it is not easy tosufficiently secure peel strength of the spot-welded portion 10.

Furthermore, in the case where heat inputted is excessive during thesubsequent energization, the HAZ hardened portion 14H is tempered.However, the nugget 12 is quenched again. Thus, although the HAZhardened portion 14H is tempered, the nugget 12 is quenched again, andhence the nugget 12 becomes hardened.

As a result, the toughness of the nugget 12 is deteriorated, and thepeel strength of the spot-welded portion 10 is reduced.

As described above, with the conventional subsequent energizationmethod, it is not easy to sufficiently obtain the effect of improvingthe toughness of the spot-welded portion, and there is a problem inwhich welding time increases, which leads to a notion that thisconventional subsequent energization method is not practical. In orderto solve these problems, various techniques have been disclosed.

Patent Document 1 discloses an invention in which conditions forsubsequent energization are determined according to sheet sets throughnumerical calculation.

Patent Document 2 discloses an invention in which subsequentenergization is applied at least once for a short period of time underhigh electric current conditions to effectively heat a portion that isto be a starting point of fracture, thereby reducing the welding time,and furthermore, the invention has a wide range of appropriateconditions.

Patent Document 3 discloses an invention that improves the fracturestrength of the joined portion by increasing the width of the HAZsoftening zone in the vicinity of the nugget through subsequentenergization, and making the structure fine while maintaining thehardness of the nugget.

Patent Document 4 discloses an invention related to spot welding thatcan secure excellent tensile strength when applied to a high-tensilesteel sheet, by forming the maximum point of hardness in a HAZ portionwhile maintaining the hardness of a nugget through spot welding with asimple two-step energization type formed by combining main energizationand tempering energization.

RELATED ART DOCUMENTS Patent Document

-   Patent Document 1: Japanese Unexamined Patent Application, First    Publication No. 2002-103054-   Patent Document 2: Japanese Unexamined Patent Application, First    Publication No. 2010-115706-   Patent Document 3: Japanese Unexamined Patent Application, First    Publication No. 2012-187617-   Patent Document 4: Japanese Unexamined Patent Application, First    Publication No. 2008-229720

Non-Patent Document

-   Non-Patent Document 1: “Tetsu-to-Hagane” Vol. 68, No. 9, P1444-1451

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

According to the technique disclosed in Patent Document 1, an objectthereof is to improve peel strength and fatigue strength of thespot-welded portion, and it is possible to optimize conditions forsubsequent energization. However, the effect obtained therefrom islimited because this technique utilizes residual stress.

According to the technique disclosed in Patent Document 2, it ispossible to soften the nugget and the hardened portion of the HAZserving as a fracture starting point to improve the toughness, byoptimizing subsequent energization after welding.

However, it does not specifically indicate the state of softness.Furthermore, although the cross-tension strength is improved, themechanism thereof is not clear, and the peel strength is not necessarilysufficiently improved.

According to the technique disclosed in Patent Document 3, although itargues that the fracture strength can be improved by increasing thewidth of the HAZ portion, the position of HAZ softening, rather than thewidth of the HAZ portion, is more important to alleviate strainconcentration as will be described later, and hence, there is apossibility that the strain concentration on the nugget end portioncannot be sufficiently alleviated.

According to the technique disclosed in Patent Document 4, excellenttensile strength is obtained by changing the distribution of thehardened portion in the HAZ portion. However, this technique improvesthe strength of a joint by distributing the strain concentrated on theHAZ portion. Thus, there is a possibility that an effect is less likelyto be obtained in the case where fracture occurs within the nugget.

Furthermore, the techniques disclosed in Patent Documents 3 and 4 aretechniques by which the effects cannot be obtained, for example, in thecase of hot-stamped members of a 1500 MPa class or higher, to which thepresent invention is directed.

As described above, in the case of the high-strength steel sheetincluding martensite, it is difficult to improve the peel strength ofthe spot-welded portion through subsequent energization, and effectivesubsequent energization methods are desired. Furthermore, in place ofthe subsequent energization having poor productivity because of longerwelding time, there has been a demand for a technique that can improvethe peel strength of the spot-welded portion through singleenergization.

The present invention has been made in view of the situations asdescribed above, and an object of the present invention is to provide anoverlap-welded member, an automobile part including the overlap-weldedmember, a method of welding an overlapped portion, and a method ofmanufacturing the overlap-welded member, which can improve the peelstrength of the spot-welded portion.

Means for Solving the Problem

Each aspect of the present invention is as follows:

(1) A first aspect of the present invention provides an overlap-weldedmember in which an overlapped portion including plural steel sheetmembers is joined at a spot-welded portion, in which at least one of theplural steel sheet members contains martensite; and the spot-weldedportion includes: a nugget formed through spot welding; a heat-affectedzone formed in the vicinity of the nugget; a softest zone having thelowest Vickers hardness in the heat-affected zone; and a tempered areaformed between a central portion of the nugget and the softest zone andmade out of tempered martensite having Vickers hardness of not more than120% in a case where Vickers hardness of the softest zone is 100%.(2) A second aspect of the present invention provides an overlap-weldedmember in which an overlapped portion including plural steel sheetmembers is joined at a spot-welded portion, in which at least one of theplural steel sheet members contains martensite; the spot-welded portionincludes a nugget formed through resistance spot welding a heat-affectedzone formed in the vicinity of the nugget, and a softest zone having thelowest Vickers hardness in the heat-affected zone; and Equation (1)described below is satisfied, where D (mm) is a distance from a meltingboundary portion of the nugget to the softest zone, and if there is onlyone steel sheet member having the highest tensile strength of the pluralsteel sheet members, t (mm) is a thickness of this steel sheet member,whereas, if there are plural steel sheet members having the highesttensile strength, t (mm) is a thickness of a steel sheet member havingthe thinnest thickness of these steel sheet members.

D≤t ^(0.2)  Equation (1)

(3) In the overlap-welded member according to (1) or (2) describedabove, the plural steel sheet members may include a hot-stamped member.(4) A third aspect of the present invention provides an automobile partincluding the overlap-welded member according to any one of (1) to (3)described above.(5) A fourth aspect of the present invention provides a method ofwelding an overlapped portion, including a resistance spot weldingprocess in which a spot-welded portion is formed through resistance spotwelding in an overlapped portion including a plurality of steel sheetmembers, the spot-welded portion including: a nugget; a heat-affectedzone formed in the vicinity of the nugget; and a softest zone having thelowest Vickers hardness in the heat-affected zone, and a temperingprocess of forming, between a central portion of the nugget and thesoftest zone, a tempered area made out of tempered martensite havingVickers hardness of not more than 120% in a case where Vickers hardnessof the softest zone is 100%.(6) In the method of welding an overlapped portion according to (5)described above, in the resistance spot welding process, energizationmay be performed so as to satisfy Equation (2) described below, where: T(second) is an energization time in the resistance spot welding; ifthere is only one steel sheet member having the highest tensile strengthof the plural steel sheet members, t (mm) is a thickness of this steelsheet member, whereas, if there are plural steel sheet members havingthe highest tensile strength, t (mm) is a thickness of a steel sheetmember having the thinnest thickness of these steel sheet members; andcyc (second) is a period of time for one cycle of energization in theresistance spot welding.

5t×cyc≤T≤(5t+4)×cyc  Equation (2)

(7) In the method of welding an overlapped portion according to (5)described above, it may be possible that the method further include,before the resistance spot welding process, applying a preheat electriccurrent I (kA) to the overlapped portion in a state where anenergization time T₁ (second), a period of time cyc (second) for onecycle of energization, and a thickness t (mm) satisfy Equation (3)described below; as for the thickness t (mm), if there is only one steelsheet member having the highest tensile strength of the plurality ofsteel sheet members, a thickness of this steel sheet member is used,whereas, if there are a plurality of steel sheet members having thehighest tensile strength, a thickness of a steel sheet member having thethinnest thickness of these steel sheet members is used; in theresistance spot welding process, a welding electric current I₀ (kA) notmore than a splash occurring current is applied to the overlappedportion in a state where Equation (4) described below is satisfied,where T₂ (second) is an energization time, and cyc (second) is a periodof time for one cycle of energization in the resistance spot welding;and the preheat electric current I (kA) and the welding electric currentI₀ (kA) satisfy Equation (5) described below.

5t×cyc≤T ₁≤(5t+8)×cyc  Equation (3)

5t×cyc≤T ₂≤(5t+4)×cyc  Equation (4)

0.3I ₀ ≤I≤0.7I ₀  Equation (5)

(8) In the method of welding an overlapped portion according to (5)described above, it may be possible that, in the resistance spot weldingprocess, the resistance spot welding be performed so as to satisfyEquation (6) described below, where D (mm) is a distance from a meltingboundary portion of the nugget to the softest zone, and if there is onlyone steel sheet member having the highest tensile strength of the pluralsteel sheet members, t (mm) is a thickness of this steel sheet member,whereas, if there are plural steel sheet members having the highesttensile strength, t (mm) is a thickness of a steel sheet member havingthe thinnest thickness of these steel sheet members, and the temperingprocess is a subsequent energization process in which the tempered areais formed through subsequent energization.

D≤t ^(0.2)  Equation (6)

(9) In the method of welding an overlapped portion according to (8)described above, in the resistance spot welding process, energizationmay be applied so as to satisfy Equation (7) described below, where T(second) is an energization time in the resistance spot welding, and cyc(second) is a period of time for one cycle of energization in theresistance spot welding.

5t×cyc≤T≤(5t+4)×cyc  Equation (7)

(10) In the method of welding an overlapped portion according to (8)described above, it may be possible that the method further include,before the resistance spot welding process: applying a preheat electriccurrent I (kA) to the overlapped portion in a state where anenergization time T₁ (second), a period of time cyc (second) for onecycle of energization, and the thickness t (mm) satisfy Equation (8)described below; in the resistance spot welding process, a weldingelectric current I₀ (kA) not more than a splash occurring current isapplied to the overlapped portion in a state where Equation (9)described below is satisfied, where T₂ (second) is an energization time,and cyc (second) is a period of time for one cycle of energization; and

the preheat electric current I (kA) and the welding electric current I₀(kA) satisfy Equation (10) described below.

5t×cyc≤T ₁≤(5t+8)×cyc  Equation (8)

5t×cyc≤T ₂≤(5t+4)×cyc  Equation (9)

0.3I ₀ ≤I≤0.7I ₀  Equation (10)

(11) A fifth aspect of the present invention provides a method ofwelding an overlapped portion including a resistance spot weldingprocess in which a spot-welded portion is formed in an overlappedportion including a plurality of steel sheet members, the spot-weldedportion including: a nugget; a heat-affected zone formed in the vicinityof the nugget; and a softest zone having the lowest Vickers hardness inthe heat-affected zone, and in the resistance spot welding process,resistance spot welding is performed so as to satisfy Equation (11)described below, where D (mm) is a distance from a melting boundaryportion of the nugget to the softest zone, and if there is only onesteel sheet member having the highest tensile strength of the pluralsteel sheet members, t (mm) is a thickness of this steel sheet member,whereas, if there are plural steel sheet members having the highesttensile strength, t (mm) is a thickness of a steel sheet member havingthe thinnest thickness of these steel sheet members.

D≤t ^(0.2)  Equation (11)

(12) In the method of welding an overlapped portion according to (11)described above, in the resistance spot welding process, energizationmay be applied so as to satisfy Equation (12) described below, where T(second) is an energization time in the resistance spot welding, and cyc(second) is a period of time for one cycle of energization in theresistance spot welding.

5t×cyc≤T≤(5t+4)×cyc  Equation (12)

(13) In the method of welding an overlapped portion according to (11)described above, it may be possible that the method further include,before the resistance spot welding process, applying a preheat electriccurrent I (kA) to the overlapped portion in a state where anenergization time T₁ (second), a period of time cyc (second) for onecycle of energization, and the thickness t (mm) satisfy Equation (13)described below; in the resistance spot welding process, a weldingelectric current I₀ (kA) not more than a splash occurring current isapplied to the overlapped portion in a state where Equation (14)described below is satisfied, where T₂ (second) is an energization time,and cyc (second) is a period of time for one cycle of energization; andthe preheat electric current I (kA) and the welding electric current I₀(kA) satisfy Equation (15) described below.

5t×cyc≤T ₁≤(5t+8)×cyc  Equation (13)

5t×cyc≤T ₂≤(5t+4)×cyc  Equation (14)

0.3I ₀ ≤I≤0.7I ₀  Equation (15)

(14) A sixth aspect of the present invention provides a method ofmanufacturing an overlap-welded member in which an overlapped portionincluding plural steel sheet members is joined at a spot-welded portion,the method including: overlapping the plural steel sheet members at aposition of the overlapped portion; and welding the overlapped portionthrough the method of welding an overlapped portion according to any oneof (5) to (13) described above.

It should be noted that, in this specification, “cyc” represents onecycle (1/frequency) (second) of power supply used for energization inthe resistance spot welding. In the case of 60 Hz, 1×cyc is ( 1/60)(second), and in the case of 50 Hz, 1×cyc is ( 1/50) (second).

Effects of the Invention

According to the overlap-welded member, the automobile part includingthe overlap-welded member, the method of welding an overlapped portion,and the method of manufacturing the overlap-welded member of the presentinvention, it is possible to improve the peel strength of thespot-welded portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating distribution of hardnessin a spot-welded portion in the case where conventional energizationconditions are applied to a TRIP member.

FIG. 2A is a perspective view schematically illustrating a cross-tensiontest.

FIG. 2B is a diagram illustrating a fracture mode concerning aspot-welded portion with a cross-tension test, and is a sectional viewillustrating a partial plug fracture.

FIG. 2C is a diagram illustrating a fracture mode concerning aspot-welded portion with a cross-section test, and is a sectional viewillustrating a plug fracture.

FIG. 2D is a diagram illustrating an example of a correlation betweentensile strength of a base metal and a cross-tension strength.

FIG. 3 is a diagram schematically illustrating a subsequent energizationmethod.

FIG. 4 is a diagram schematically illustrating distribution of hardnessin a spot-welded portion after the spot-welded portion is formed byoverlapping test pieces according to a subsequent energization methodillustrated in FIG. 3, and applying subsequent energization.

FIG. 5 is a diagram schematically illustrating changes of a HAZ in aspot-welded portion in the case where the spot-welded portion formed ona hot-stamped member is subjected to subsequent energization.

FIG. 6 is a diagram illustrating a schematic configuration of aspot-welded portion including a nugget according to an embodiment of thepresent invention.

FIG. 7 is a diagram illustrating a schematic configuration of a nuggetand a HAZ in the same spot-welded portion.

FIG. 8 is a diagram explaining energization conditions in resistancespot welding according to an embodiment of the present invention.

FIG. 9A is a diagram schematically illustrating portions of aspot-welded portion according to an embodiment of the present inventionwhere hardness is measured.

FIG. 9B is a graph showing a relationship between distances (mm) from amelting boundary of a nugget and Vickers hardness.

FIG. 10A is a diagram illustrating an analysis model under a short-timeenergization condition with a distance from a nugget end to the softestzone in HAZ being set to 0.75 mm.

FIG. 10B is a diagram illustrating an analysis model under a normalcondition with a distance from a nugget end to the softest zone in HAZbeing set to 1.5 mm.

FIG. 11 is a graph related to an analysis model of each spot-weldedportion under “(a) short-time energization condition,” “(b) normalcondition,” and “(c) no HAZ softening,” and illustrating equivalentplastic strain at Position 1 illustrated in FIG. 10A.

FIG. 12 is a graph related to an analysis model of each spot-weldedportion under “(a) short-time energization condition,” “(b) normalcondition,” and “(c) no HAZ softening,” and illustrating equivalentplastic strain at Position 2 illustrated in FIG. 10A.

FIG. 13A is a diagram illustrating a relationship between a thickness tand a distance D from a melting boundary of a nugget to a HAZ softeningzone.

FIG. 13B is a diagram illustrating a relationship between cross-tensionstrength and distances D from the melting boundary of a nugget to a HAZsoftening zone.

FIG. 14 is a graph showing behavior of nugget growth in the case where ashort-time energization condition, a normal condition, and a two-stepenergization condition are applied to a hot-stamped member of an 1800MPa class having a thickness of 1.6 mm.

FIG. 15 is a graph showing distribution of hardness in a spot-weldedportion formed under the conditions shown in FIG. 14.

FIG. 16 is a diagram schematically illustrating changes of HAZ inspot-welded portions after single energization and after subsequentenergization in the case where a short-time energization conditionaccording to an embodiment of the present invention is applied to ahot-stamped member.

FIG. 17 is a diagram schematically illustrating changes in Vickershardness in spot-welded portions after single energization and aftersubsequent energization in the case where a short-time energizationcondition according to an embodiment of the present invention is appliedto a hot-stamped member.

FIG. 18 is a graph showing distribution of hardness in spot-weldedportions after single energization in the case where a short-timeenergization condition according to an embodiment of the presentinvention and a normal energization condition are applied to ahot-stamped member of an 1800 MPa class having a thickness of 1.8 mm.

FIG. 19 is a graph showing distribution of hardness in spot-weldedportions after subsequent energization in the case where a short-timeenergization condition according to an embodiment of the presentinvention and a normal energization condition are applied to ahot-stamped member of an 1800 MPa having a thickness of 1.8 mm.

FIG. 20 is a diagram schematically illustrating changes of distributionof hardness in a spot-welded portion obtained by forming the spot-weldedportion under a short-time energization condition according to anembodiment of the present invention and a normal energization condition,and then, applying subsequent energization.

FIG. 21 is a perspective view illustrating an L-shaped test.

EMBODIMENTS OF THE INVENTION

The present inventors carried out thorough investigations on improvingthe peel strength in the case where plural steel sheet members includingat least one steel sheet member containing martensite are joined at aspot-welded portion in an overlapped portion. As a result, it was foundthat, by applying single energization (short-time single energization)under a short-time energization condition in which an electric currentvalue is increased and an energization period of time is shorter thanconventional one, the HAZ hardened portion is reduced, and a distancebetween a nugget end and the softest zone in HAZ is reduced.

Furthermore, it was also found that, with the reduction in the distancebetween the nugget end and the softest zone in HAZ, the stress acting atthe time of load applied on a nugget end portion area in the peelingdirection is alleviated, and the peel strength largely improves.

On the basis of the findings described above, in place of the subsequentenergization with the conventional type that needs longer time, thepresent inventors developed a method that can improve the strength withsingle energization.

Furthermore, it was also found that, by reducing the distance betweenthe nugget end and the softest zone in HAZ, and applying the subsequentenergization, the nugget and the HAZ hardened portion are tempered,whereby it is possible to suppress hard portions being locally formedbetween the nugget end and the softest zone in HAZ. Thus, the peelstrength of the spot-welded portion is improved as compared with thesubsequent energization with the conventional type.

Below, the present invention made on the basis of the findings describedabove will be described in detail with reference to the drawings.

FIG. 6 is a sectional view illustrating a spot-welded portion, whichillustrates the schematic configuration of a spot-welded portion 10formed in an overlap-welded member used, for example, as an automobilepart, according to an embodiment of the present invention.

The overlap-welded member according to this embodiment is formed byjoining steel sheet members S1 and S2 through a spot-welded portion 10as illustrated in FIG. 6.

As illustrated in FIG. 6, a nugget 12 is formed in an overlapped portionof the steel sheet members S1 and S2 through energization applied from apair of electrodes 50, 50, which are used for resistance spot weldingand to squeeze the steel sheet members S1 and S2 in the thicknessdirection between the pair of electrodes with the central line CL ofelectrodes 50 being the center.

As for molten metal generated through the energization, solidificationgrows in an area in the vicinity of the central line CL and in contactwith the electrodes 50 toward the thickness direction due to heatdissipated to the electrodes 50, whereas, in an area distant from thecentral line CL of the electrodes 50, solidification grows toward thecentral direction of the nugget (toward the central line CL of theelectrodes) in addition to toward the thickness direction.

As a result, the nugget 12 includes an area 12A where dendrite grows inthe thickness direction, and an area 12B where dendrite grows so as tointersect the thickness direction.

In this specification, when the overlapped portion is viewed from thethickness direction, the nugget end 12E represents the outermostboundary (in other words, melting boundary portion of the nugget 12)that melts when the nugget 12 is formed, and the nugget end portion area12B represents an area from a meeting portion 12C between the area 12Aand the area 12B to the nugget end 12E.

FIG. 7 is a sectional view illustrating a spot-welded portion, whichillustrates the spot-welded portion 10 where the overlapped portion iswelded. The spot-welded portion 10 includes the nugget 12 formed throughspot welding, and the HAZ 14 formed in the vicinity of this nugget 12through spot welding.

The HAZ 14 includes a HAZ hardened portion 14H formed next to the nugget12, and a HAZ softening zone 14T formed around the HAZ hardened portion14H.

Furthermore, the softest zone 14L in HAZ having the lowest Vickershardness is formed in the vicinity of the inner peripheral portion inthe HAZ softening zone 14T.

The reference character D illustrated in FIG. 7 represents a distancebetween the nugget end 12E and the softest zone 14L in HAZ.

FIG. 8 is a diagram illustrating energization conditions in resistancespot welding according to this embodiment.

As illustrated in FIG. 8, in the case of a short-time energizationcondition C11 according to this embodiment, resistance spot welding isperformed by first applying single energization in which an energizationelectric current I11, which is higher than an energization electriccurrent I21 under a normal energization condition C21, is applied for anenergization time T11, which is shorter than a conventional normalenergization time T21.

The broken line in FIG. 8 indicates the first energization C21 (anelectric current value I21 and an energization time T21) under a normalcondition, where the electric current value I11>the electric currentvalue I21, and the energization time T11 (cyc)<the energization time T21(cyc).

Furthermore, in FIG. 8, the reason that the short-time energizationcondition C11 is illustrated from the intermediate stage in the normalenergization condition C21 on the time axis is to match the completiontimes of energization.

In the short-time energization condition C11 according to thisembodiment, as illustrated in FIG. 8, the molten metal generated at thetime of forming the nugget 12 through energization is rapidly cooledafter the single energization is completed, and temperatures thereofpass through the Ms point and are decreased to the Mf point or lower, sothat martensite is formed.

Furthermore, by comparing a temperature curve H1 of the nugget 12 underthe short-time energization condition C11 with a temperature curve H2 ofthe nugget under the normal energization condition C21, the joinedportion is melted, and the nugget 12 is formed under the short-timeenergization condition C11 in a shorter period of time than those underthe normal energization condition C21.

Thus, with the short-time energization condition C11, the excessive heatflow to the vicinity of the nugget 12 is suppressed, the size of the HAZhardened portion is reduced, and the distance D between the nugget end12E and the softest zone 14L in HAZ is reduced.

As a result, only with the single energization described above are thestrains at the time of peeling concentrated on portions other than thenugget end portion area 12B, and the stress concentrated on the nuggetend portion area 12B can be alleviated, whereby the peel strengthimproves.

It should be noted that the spot-welded portion 10 formed with singleenergization under the short-time energization condition C11 may be usedas it is without applying additional processing. Furthermore, after apredetermined suspension period of time Ts elapses, it may be possibleto apply subsequent energization (in other words, the secondenergization) under a subsequent energization condition C12 to thespot-welded portion 10 thus formed.

By applying the energization under the subsequent energization conditionC12 (an electric current value 112 and an energization time T12) to thespot-welded portion 10 formed under the short-time energizationcondition C11 after energization is suspended for the suspension periodof time Ts, the nugget 12 is heated to temperatures not less than atemperature (approximately 550 to 600° C.) at which tempering ispossible and not more than Ac₁, and then, is gradually cooled, wherebytempered martensite can be obtained without re-quenching the HAZ 14.

As described above, in the case of the short-time energization conditionC11, the electric current value I11 is set so as to be larger than theelectric current value I21 under the normal energization condition C21,and the energization time T11 is set so as to be shorter than theenergization time T21 under the normal energization condition C21. Thus,temperatures of the nugget 12 are raised in a short period of time, andtransference of the heat generated through energization to the vicinitythereof is not developed, whereby the HAZ 14 is less likely to becomehigh temperature as compared with the normal condition.

As a result, it can be considered that the width of the HAZ hardenedportion 14H is narrow, and the distance D between the nugget end 12E andthe softest zone 14L in HAZ is reduced.

As described above, by forming the nugget 12 under the short-timeenergization condition C11, the width of the HAZ hardened portion 14Hcan be made narrow, whereby the nugget 12 and the HAZ hardened portion14H are sufficiently tempered.

Thus, it is possible to prevent high Vickers hardness portions frombeing formed between the nugget end 12E and the softest zone 14L in HAZ.

More specifically, since the HAZ hardened portion 14H is softened in auniform manner, deformation becomes easy, and stress acting on thenugget end portion area 12B at the time of peeling is reduced, wherebyit is possible to improve the peel strength.

As described above, by employing the short-time energization conditionC11 and the subsequent energization condition C12, Vickers hardnessbetween the softest zone 14L in HAZ and the nugget end 12E can be madeto 120% or lower in the case where Vickers hardness in the softest zone14L in HAZ is 100%, whereby toughness of the spot-welded portion 10 canbe sufficiently secured.

Below, a relationship between distances (mm) from the melting boundaryof the nugget 12 and Vickers hardness will be described with referenceto FIG. 9A and FIG. 9B.

FIG. 9A and FIG. 9B are diagrams each illustrating a case where a firstenergization is applied to a hot-stamped member of a 1500 MPa classhaving a thickness of 1.6 mm under a “(a) short-time energizationcondition” and a “(b) normal condition” according to this embodiment.FIG. 9A is a sectional view illustrating a spot-welded portion, and FIG.9B is a graph showing distribution of hardness in the spot-weldedportion 10.

As for measurement of distribution of hardness, as illustrated in FIG.9A, measurement is performed at a position located at a distance of ¼ ofthe thickness from the joining surface of the steel sheet members S1 andS2 toward the steel sheet member S1 and the inner side of the steelsheet member S1 by applying a load of 9.8 N at 0.5 mm pitches accordingto JIS Z 2244.

In the graph shown in FIG. 9B, the “blank diamonds” represent theshort-time energization condition, and the “blank circles” represent thenormal energization condition.

It should be noted that the energization time in the short-timeenergization condition is set to 9×cyc, the energization time in thenormal condition is set to 20×cyc, and the electric current value isadjusted such that the nugget diameter is 4√t (mm) (t represents athickness).

From FIG. 9A and FIG. 9B, it can be understood that the distance D fromthe nugget end 12E to the softest zone 14L in HAZ is reduced by applyingthe first energization under the “(a) short-time energizationcondition.”

Below, equivalent strain in the case where energization is performedunder the short-time energization and the normal condition will bedescribed with reference to FIG. 10A, FIG. 10B, FIG. 11, and FIG. 12.

Equivalent plastic strains in the case of the “(a) short-timeenergization condition,” the “(b) normal condition,” and the “(c) no HAZsoftening” are obtained through elasto-plastic FEM analysis under theshort-time energization and the normal condition. Detailed descriptionswill be made below.

FIG. 10A is a sectional view illustrating a spot-welded portion, whichillustrates an analysis model of a test piece for single energizationobtained by applying the first energization under the “(a) short-timeenergization condition” with the distance D from the nugget end 12E tothe softest zone 14L in HAZ being set to 0.75 mm.

FIG. 10B is a sectional view illustrating a spot-welded portion, whichillustrates an analysis model of a test piece for single energizationobtained by applying the first energization under the normal conditionwith the distance D from the nugget end 12E to the softest zone 14L inHAZ being set to 1.5 mm.

It should be noted that, for the analysis models, the distribution ofhardness in the HAZ softening zone is varied in a stepwise manner fromthe hardness of the softest zone to the hardness of the base metalportion on the basis of the measurement results shown in FIG. 9B.

In FIG. 10A and FIG. 10B, the Position 1 represents the softest zone 14Lin HAZ, and the Position 2 represents the nugget end 12E.

For the analysis models, three patterns are used, which include a casewhere the “(a) short-time energization condition” in FIG. 10A issimulated, a case where the “(b) normal condition” in FIG. 10B issimulated, and a case where the “(c) no HAZ softening” is simulated.

FIG. 11 is a graph showing equivalent plastic strains at the Position 1illustrated in FIG. 10A in the case where the analysis models formed bythe spot-welded portions with the “(a) short-time energizationcondition,” the “(b) normal condition,” and the “(c) no HAZ softening”are subjected to cross-tension testing with a load with which thespot-welded portion in the case of the “(b) normal condition” isfractured through the cross-tension testing.

It should be noted that, in FIG. 11, the Position 1 in the analysismodel with the “(c) no HAZ softening” is set to the same position asthat with the “(b) normal condition.”

As shown in the graph in FIG. 11, the equivalent plastic strain at thePosition 1 is approximately 0.032 with the “(a) short-time energizationcondition,” which significantly increases as compared with 0.013 withthe “(b) normal condition” and approximately 0.018 with the “(c) no HAZsoftening.”

FIG. 12 is a graph showing equivalent plastic strains at the Position 2illustrated in FIG. 10A in the case where the analysis models formed bythe spot-welded portions with the “(a) short-time energizationcondition,” the “(b) normal condition,” and the “(c) no HAZ softening”are subjected to cross-tension testing with a load with which thespot-welded portion in the case of the (b) normal condition is fracturedthrough the cross-tension testing.

It should be noted that, in FIG. 12, the Position 2 in the analysismodel with the “(c) no HAZ softening” is set to the same position asthat with the “(b) normal condition.”

Furthermore, as shown in the graph in FIG. 12, the equivalent plasticstrain at the Position 2 is approximately 0.010 with the “(a) short-timeenergization condition,” which decreases as compared with 0.0115 withthe “(b) normal condition” and approximately 0.0118 with the “(c) no HAZsoftening.”

However, at the position of HAZ softening with the “(b) normalcondition,” the existence or absence of the HAZ softening has a limitedeffect on the equivalent plastic strain in the end portion area of thenugget, as compared with the “(b) normal condition” and the “(c) no HAZsoftening.”

More specifically, in the case of the normal condition, the HAZsoftening zone 14T provides little effect as to reducing the strains tothe nugget end portion area 12B at the time of peeling, and since theHAZ softening zone 14T approaches the nugget end portion area 12B, thestrains concentrate on the HAZ softening zone 14T. As a result, it wasfound that the strains concentrated on the nugget end portion area 12Bcan be reduced. In other words, with this effect, the peel strength canbe increased by using the “short-time energization condition.”

Below, conditions appropriate for the overlap-welded member obtained byjoining, at the spot-welded portion, overlapped portions includingplural steel sheet members, according to this embodiment will bedescribed with reference to FIG. 13A and FIG. 13B.

FIG. 13A is a diagram illustrating a relationship between the thicknesst (mm) of the overlapped portion and a distance D (mm) from a meltingboundary (end of the nugget) of the nugget to the softest zone in HAZ.

In FIG. 13A, the “blank circles” represent a DP steel of a 980 MPa classwith a conventional single energization.

Furthermore, the “blank diamonds” represent a hot-stamp steel of a 1500MPa class with a conventional single energization.

Here, as for the thickness t (mm) in FIG. 13A, in the case where thereis only one steel sheet member having the highest tensile strength ofplural steel sheet members, t (mm) is the thickness of this steel sheetmember, and in the case where there are plural steel sheet membershaving the highest tensile strength, t (mm) is the thickness of a steelsheet member having the thinnest thickness of all the steel sheetmembers.

As illustrated in FIG. 13A, in the case of the single energization witha conventional condition, the distance D between the nugget end 12E andthe softest zone 14L in HAZ on the overlapping interface between twosteel sheets is formed so as to fall in a range exceeding D (mm)=t^(0.2)(mm). At the time of the cross-tension test, in the case of a sheet setobtained by combining a steel type having low joint strength and a steeltype having high joint strength, fracture tends to occur on the side ofthe low joint strength.

For example, in the case where the strength of the base metal is higherthan a 780 MPa class, the cross-tension strength decreases with anincrease in the strength of the base metal, and hence, fracture is morelikely to occur as the strength of the base metal increases.

Furthermore, in the case of a sheet set having the same steel type butdifferent thicknesses, fracture occurs on the side of the steel sheethaving the thinner thickness.

For the reasons described above, the thickness t of a steel sheet memberhaving the thinnest thickness is employed.

FIG. 13B is a diagram illustrating a relationship between thecross-tension strength and the distance D from the nugget end 12E of thespot-welded portion to the HAZ softening zone in the case where thenugget diameter in a hot-stamped member of a 1500 MPa class is 4√t.

As illustrated in FIG. 13B, by setting the distance D (mm) between thenugget end 12E and the softest zone 14L in HAZ to t^(0.2) (mm) orshorter, the cross-tension strength increases to approximately 7 kN andis made stable, which makes it possible to make the fracture mode to bethe plug fracture. Furthermore, by setting the distance D (mm) betweenthe nugget end 12E and the softest zone 14L in HAZ to 0.75×(t^(0.2))(mm) or shorter, the cross-tension strength increases to approximately 8kN, and is made further stable, which makes the fracture mode to be theplug fracture. This is more favorable.

As described above, by reducing the distance D from the nugget end 12Eof the spot-welded portion to the HAZ softening zone 14T, thecross-tension strength improves.

Furthermore, as for the hardness from the base metal toward the nuggetend portion area 12B (including the nugget end portion area 12B) on theoverlapping interface between these two steel sheet members, thehardness gradually decreases toward the nugget end 12E in a range wherethe maximum value of Vickers hardness relative to the softest zone 14Lin HAZ is approximately 115%, or the hardness is equivalent to thehardness of the softest zone 14L in HAZ.

According to the overlap-welded member of this embodiment, by bringingthe HAZ softening zone 14T close to the nugget end portion area 12B, thestress concentration on the nugget end portion area 12B serving as thestarting point of the fracture within the nugget (interface fracture,partial plug fracture) is alleviated, whereby it is possible to improvethe joint strength.

The effect of improving the joint strength becomes more apparent as thefracture mode changes from the fracture within the nugget (the interfacefracture and the partial plug fracture) to the plug fracture.

In particular, as for a joint for which the plug fracture cannot beobtained because the toughness of the nugget 12 itself is not sufficientand the crack propagates into the nugget even if the stressconcentration on the nugget end portion area 12B is alleviated, byapplying subsequent energization to this joint in addition tooptimization of the HAZ softening zone 14T, it is possible to obtain theeffect of improving the joint strength stronger than the conventionalone.

This mechanism has already been described above.

As described above, if the distance D (mm) from the nugget end 12E tothe softest zone 14L in HAZ satisfies

D≤t ^(0.2)  Equation (1),

it is possible to sufficiently improve the joint strength.

Thus, with the overlap-welded member according to this embodiment, acondition is set such that the distance D from the nugget end 12E to thesoftest zone 14L in HAZ satisfies Equation (1) described above.

Furthermore, by making the distance D (mm) from the nugget end 12E tothe softest zone in HAZ satisfy

D≤0.75×(t ^(0.2))  Equation (1A),

the fracture mode can be more reliably made to be the plug fracture,which is preferable.

Below, a method of welding the overlap-welded portion using a resistancespot welding process and a tempering process will be described indetail.

(Resistance Spot Welding Process)

In the resistance spot welding process, a spot-welded portion 10including a nugget 12, a HAZ 14 formed around this nugget 12, and thesoftest zone 14L having the lowest Vickers hardness in this HAZ 14 isformed through resistance spot welding at an overlapped portion formedby plural steel sheet members.

(Tempering Process)

In the tempering process, a tempered area made out of temperedmartensite having the Vickers hardness of 120% or lower in the casewhere the Vickers hardness of the softest zone 14L is 100% is formedbetween the central portion of the nugget 12 formed through theresistance spot welding process and the softest zone 14L.

It is preferable to apply subsequent energization to form the temperedarea. However, this formation is not limited to through the subsequentenergization. It may be possible to use, for example, emission of laserbeam to form the tempered area.

With the method of welding an overlapped portion according to thisembodiment as described above, the tempered area having the Vickershardness of 120% or lower in the case where the Vickers hardness of thesoftest zone 14L is 100% is formed between the central portion of thenugget 12 and the softest zone 14L.

Furthermore, in the resistance spot welding process described above, thenugget 12 may be formed with an energization time T expressed in thefollowing manner, where t (mm) is the thickness, and cyc (second) is aperiod of time for one cycle of energization in the resistance spotwelding.

5t×cyc≤T≤(5t+4)×cyc  Equation (2)

In general, in the spot welding, with an increase in the thickness, theenergization time increases, and the distance D from the nugget end 12Eto the softest zone 14L in HAZ tends to increase. However, with thesatisfaction of this Equation (2), the nugget can be stably formed, andthe distance D (mm) between the nugget end 12E and the softest zone 14Lin HAZ can be more reliably formed so as to be not more than t^(0.2).

In other words, it is possible to stably improve the peel strength atthe spot-welded portion.

It should be noted that, as for the thickness t (mm), in the case wherethere is only one steel sheet member having the highest tensile strengthof plural steel sheet members, t (mm) is the thickness of this steelsheet member, and in the case where there are plural steel sheet membershaving the highest tensile strength, t (mm) is the thickness of a steelsheet member having the thinnest thickness of all the steel sheetmembers.

(Preheat Energization Process)

As described above, by applying spot welding while satisfying theenergization time specified in this embodiment, this spot welding iseffective from the viewpoint of the HAZ softening zone 14T. On the otherhand, the appropriate electric current range reduces as compared withthe conventional energization condition.

In this respect, the present inventors found that it is preferable toperform a preheat energization process before the resistance spotwelding process described above is performed, in terms of being able tobring the softest zone in HAZ closer to the end portion area of thenugget as compared with the conventional technique while maintaining theappropriate electric current range (margin of electric current to thesplash occurring current) equivalent to the conventional condition.

Here, the above-described effect obtained by performing the preheatenergization process will be described with reference to FIG. 14 andFIG. 15.

FIG. 14 is a graph showing behavior of nugget growth in the case wherethe short-time energization condition (9×cyc), the normal condition(20×cyc), and a two-step energization condition (energization time inthe first step: 11×cyc, welding electric current: 4 kA, and energizationtime in the second step: 9×cyc) are applied to a hot-stamped member ofan 1800 MPa class having a thickness of 1.6 mm.

Furthermore, FIG. 15 is a graph showing distribution of Vickers hardnessbased on distances from the nugget end of the spot-welded portion formedunder the conditions shown in FIG. 14.

As shown in FIG. 14 and FIG. 15, by applying two-step energizationincluding the preheat energization and the main energization, it ispossible to bring the position of HAZ softening closer to the nugget endportion area 12B as compared with the conventional technique whilemaintaining the appropriate electric current range almost equivalent tothat of the conventional technique.

Below, energization conditions for the preheat energization process willbe described in detail.

In the preheat energization process, energization with preheat electriccurrent I (kA) is applied to the overlapped portion in a manner suchthat energization time T₁ (second), a period of time cyc (second) forone cycle of energization, and a thickness t (mm) satisfy

5t×cyc≤T ₁≤(5t+8)×cyc  (Equation 3).

Then, in the case where the preheat energization process is performed,the nugget is formed by, after the preheat energization process,applying energization with welding electric current I₀ (kA), which isless than or equal to the splash occurring current, to the overlappedportion so as to satisfy

5t×cyc≤T ₂≤(5t+4)×cyc  Equation (4),

where T₂ (second) is an energization time and cyc (second) is a periodof time for one cycle of energization in the resistance spot welding.

Here, a relationship between the preheat electric current I (kA) and thewelding electric current I₀ (kA) satisfies

0.3I ₀ ≤I≤0.7I ₀  Equation (5).

In the preheat energization process described above, the energizationtime T₁ (second) is longer than or equal to 5t×cyc, and the preheatelectric current I (kA) is more than or equal to 0.3I₀, in other words,is more than or equal to 30% of the welding electric current I₀ in theresistance spot welding process for forming the nugget. Thus, thepreheating effect is sufficient, and it is possible to secure a desiredappropriate electric current range, which is preferable.

Furthermore, the energization time T₁ (second) is less than or equal to(5t+4)×cyc, and the preheat electric current I (kA) is less than orequal to 0.7I₀, in other words, is less than or equal to 70% of thewelding electric current I₀ in the resistance spot welding process forforming the nugget. Thus, it is possible to reduce the distance D fromthe nugget end 12E to the softest zone 14L in HAZ, which is preferable.

Then, in the resistance spot welding process performed after the preheatenergization, the energization time T₂ is set to be not shorter than5t×cyc and not longer than (5t+4)×cyc. Thus, it is possible tosufficiently form the nugget, and it is possible to make the distance D(mm) between the nugget end 12E and the softest zone 14L in HAZ to benot longer than t^(0.2). This makes it possible to stably improve thepeel strength in the spot-welded portion.

Furthermore, by adjusting the energization time such that the D (mm) isshorter than or equal to 0.75×(t^(0.2)), it is possible to more reliablyobtain the spot-welded portion whose fracture mode is the plug fracture,and it is possible to improve the peel strength.

By applying the tempering process described above (for example,tempering with the subsequent energization) to the thus obtainedoverlap-welded portion so that the nugget end portion area 12B istempered, it is possible to form, between the central portion of thenugget 12 and the softest zone 14L, the tempered area formed by thetempered martensite having the Vickers hardness of 120% or less in thecase where the Vickers hardness of the softest zone 14L is 100%.

Thus, it is possible to manufacture the overlap-welded member having thenugget diameter same as the conventional one, exhibiting excellentstrength, and increased joint strength.

In order to obtain the effect as described above, it is necessary toadjust the short-time energization condition such that the nugget end12E and the softest zone 14L in HAZ are brought closer to each other,and then, apply the tempering process to make the Vickers hardness ofthe tempered area to be not more than 120% of the Vickers hardness ofthe softest zone 14L. However, in order to obtain the effect in a morefavorable manner, it is preferable to make the Vickers hardness of thetempered area to be not more than 115% of the Vickers hardness of thesoftest zone 14L, and it is more preferable to make the Vickers hardnessof the tempered area to be not more than 110% of the Vickers hardness ofthe softest zone 14L.

It should be noted that the lower limit value of the Vickers hardness ofthe tempered area is not specified.

If the tensile strength of the base material is stronger than or equalto a 980 MPa class, the interface fracture or the partial plug fractureis more likely to occur in the overlap-welded member, and the jointstrength tends to decrease.

This embodiment is effective to the steel sheet member having the HAZsoftening made as a result of spot welding. However, it is preferable toapply this embodiment to a high-tensile steel sheet having the basemetal with a tensile strength of a 980 MPa class or higher.

In particular, in the case of the hot-stamped member, the base metal isfull martensite. Thus, the amount of softening of HAZ is large, and theeffect obtained by this embodiment is significant.

Furthermore, as for the overlap-welded member according to thisembodiment, there is no limitation on the thickness of, the type (forexample, DP, TRIP, and so on) of, and the existence or absence ofplating of each steel sheet member in the overlapped portion formed bytwo or more steel sheet members.

Furthermore, in Example described later, although description will bemade of a sheet set obtained by overlapping two steel sheets with thesame type, application is not limited to this sheet set. The effect canbe obtained in the case of a sheet set with different materials, or asheet set with three or more sheets.

FIG. 16 is a diagram illustrating a schematic configuration of thespot-welded portion 10 formed with the energization condition accordingto this embodiment in the case where hot-stamped members are used as thesteel sheet members S1 and S2. More specifically, (a) in FIG. 16 is asectional view illustrating a spot-welded portion after a short-timeenergization is applied, and (b) in FIG. 16 is a sectional viewillustrating a spot-welded portion after a subsequent energization isapplied. (c) in FIG. 16 is a graph showing distribution of Vickershardness after the single energization and after the subsequentenergization.

Furthermore, FIG. 17 is a diagram schematically illustrating changes inVickers hardness in the spot-welded portion after the singleenergization and after the subsequent energization.

By applying the short-time energization, the distance D between thenugget end 12E and the softest zone 14L in HAZ is reduced toapproximately 1 mm as illustrated in (a) in FIG. 16.

Since the distance D between the nugget end 12E and the softest zone 14Lin HAZ through energization under a conventional normal condition isapproximately 1.5 mm, the distance is significantly reduced as comparedwith the conventional one.

As a result, it is possible to alleviate the stress concentration in thevicinity of the nugget end 12E.

Moreover, by further applying the subsequent energization, it ispossible to sufficiently temper the HAZ hardened portion 14H in thedot-hatched area illustrated in (b) in FIG. 16.

As described above, in the case where the spot-welded portion 10 isformed by applying the energization condition according to the presentinvention to the hot-stamped members S1 and S2, the Vickers hardness ofthe HAZ hardened portion 14H is almost equal to that of the nugget 12 byapplying the short-time single energization as illustrated in (a) and(b) in FIG. 17.

Furthermore, by applying the subsequent energization, the nugget 12 andthe HAZ hardened portion 14H are sufficiently tempered as illustrated in(c) in FIG. 17, and the hardness in terms of Vickers hardness betweenthe softest zone 14L in HAZ and the nugget end 12E is similar to that ofthe softest zone 14L in HAZ, or the maximum value of the hardnessthereof is approximately 115% of the softest zone 14L in HAZ. Thus, thestress in the nugget end portion area 12B is sufficiently alleviated.

As a result, it is possible to improve the peel strength of thespot-welded portion 10 of the hot-stamped members. Note that the entirenugget 12 does not necessarily have to be tempered, provided that thenugget end portion area 12B is tempered.

As described above, by tempering the HAZ hardened portion 14H, themaximum hardness in terms of Vickers hardness between the nugget 12 andthe softest zone 14L in HAZ is made fall in a range of less than orequal to approximately 120% of the hardness of the softest zone 14L inHAZ as illustrated in (c) in FIG. 16.

As a result, the toughness of the nugget 12 and the HAZ hardened portionis improved, whereby it is possible to improve the peel strength.

It should be noted that, even if the entire nugget 12 is not tempered,the joint strength improves, provided that the maximum Vickers hardnessbetween the nugget end 12E and the softest zone 14L in HAZ is less thanor equal to 120% of the Vickers hardness of the softest zone 14L in HAZ,preferably is less than or equal to 115%, more preferably is less thanor equal to 110%.

It should be noted that it is preferable that: the short-timeenergization condition be adjusted; the nugget end and the softest zonein HAZ be brought closer to each other; and the maximum value in termsof Vickers hardness between the central portion of the nugget 12 and thesoftest zone 14L in HAZ after the subsequent energization is applied bemade to be 115% relative to the softest zone 14L in HAZ.

Furthermore, it is more preferable that: the short-time energizationcondition be adjusted; the nugget end and the softest zone in HAZ bebrought close to each other; and the maximum value in terms of Vickershardness between the nugget and the softest zone in HAZ after thesubsequent energization is applied be made to be 110% relative to thesoftest zone 14L in HAZ.

FIG. 18 is a graph showing distribution of hardness in the spot-weldedportion through single energization in the case where the short-timeenergization condition according to this embodiment and a normalenergization condition are applied to a hot-stamped member of an 1800MPa class having a thickness of 1.8 mm.

Furthermore, FIG. 19 is a graph showing distribution of hardness in thespot-welded portion after the subsequent energization is applied in thecase where the short-time energization condition according to thisembodiment and a normal energization condition are applied to ahot-stamped member of an 1800 MPa class having a thickness of 1.8 mm.

In FIG. 18, the “blank diamonds” represent the distribution of hardnessin a spot-welded portion in the case where the spot-welded portion isformed with a main energization employing a short-time energizationcondition with an energization time being 9×cyc (second). Furthermore,the “blank circles” represent the distribution of hardness in aspot-welded portion in the case where the spot-welded portion is formedwith a main energization employing a normal condition with anenergization time being 22×cyc (second).

In FIG. 19, the “blank diamonds” represent the distribution of hardnessin a spot-welded portion in the case where the spot-welded portion isformed with a main energization employing a short-time energizationcondition with an energization time being 9×cyc (second), and then,tempering is performed through a subsequent energization, and the “blankcircles” represent the distribution of hardness in a spot-welded portionin the case where the spot-welded portion is formed with a mainenergization employing a normal condition with an energization timebeing 22×cyc (second), and then, tempering is performed through asubsequent energization.

First, as shown in the graph in FIG. 18, with the short-timeenergization condition indicated by the “blank diamonds,” the distancefrom the nugget end to the softest zone in HAZ is reduced, as comparedwith that of the normal condition indicated by the “blank circles.”

As shown in the graph in FIG. 19, in the case where the subsequentenergization is applied, in the spot-welded portion formed by applyingthe normal condition at the time of the main energization, hard portionsexist between the nugget and the softest zone (at a positionapproximately 1 mm from the end portion area of the nugget) because thesoftest zone in HAZ formed at the time of the main energization is farfrom the nugget end portion.

On the other hand, in the spot-welded portion formed by applying theshort-time energization condition at the time of the main energization,the softest zone in HAZ formed at the time of the main energization isbrought close to the end portion area of the nugget, and hence, it ispossible to make the Vickers hardness of the nugget and the HAZextending from the base metal to the nugget end portion area 12B(including the nugget end portion area 12B) to be 120% or lower of thehardness of the softest zone in HAZ.

In other words, in the case where the subsequent energization isapplied, the strain concentration in the vicinity of the nugget end 12Ecan be alleviated if the distance from the nugget end 12E to the softestzone in HAZ formed at the time of the main energization is reduced.

As described above, reducing the energization time for forming thenugget 12 is effective from the viewpoint of bringing the position ofthe HAZ softening closer to the end portion area of the nugget toimprove the joint strength.

FIG. 20 is a diagram schematically illustrating changes in HAZ of thespot-welded portion after the single energization and the subsequentenergization in the case where the short-time energization conditionaccording to this embodiment is applied to a DP member or a TRIP member.

(a) in FIG. 20 is a sectional view illustrating a spot-welded portion.

As illustrated in (b) in FIG. 20, in the case where the DP members orTRIP members are used as steel sheet members S1 and S2, the HAZ hardenedportion 14H has the Vickers hardness almost equal to the nugget 12 in astate where the short-time single energization is applied.

In this case, in the nugget 12 and the HAZ hardened portion 14H, thedistribution of hardness is different from the hot-stamped memberillustrated in FIG. 17 in that they are significantly harder than thebase material of the DP member or TRIP member.

Furthermore, as illustrated in (c) in FIG. 20, by applying thesubsequent energization, the nugget 12 and the HAZ hardened portion 14Hare sufficiently tempered, and the hardness in terms of Vickers hardnessbetween the softest zone 14L in HAZ and the nugget end 12E becomesapproximately 115% of the softest zone 14L in HAZ, whereby the stress inthe nugget end portion area 12B is sufficiently alleviated.

As a result, it is possible to improve the peel strength in thespot-welded portion 10 in the case of the DP member or TRIP member.

It should be noted that the entire nugget 12 does not have to benecessarily tempered, provided that the nugget end portion area 12B istempered.

It should be noted that the long dashed double-short dashed lineillustrated in (c) in FIG. 20 indicates the distribution of hardnessbefore the subsequent energization is applied.

It should be noted that, in the embodiment described above, descriptionshave been made of a case where the HAZ hardened portion 14H is temperedthrough subsequent energization. However, the nugget and the HAZhardened portion may be tempered, for example, by laser emission afterthe spot-welded portion 10 is formed through the short-time singleenergization.

EXAMPLES

Below, it is confirmed that bringing the softest zone in HAZ close tothe nugget end portion is effective to improve the joint strength in thepeeling direction.

By reducing the energization time, it is possible to bring the softestzone in HAZ close to the nugget end portion. However, from the viewpointof formation of the nugget, if the energization time is reduced, theappropriate electric current range (in general, the electric currentrange from an electric current value with which the nugget diameter of4√t can be obtained, to occurrence of splash) becomes narrow.

Then, a study was made to achieve both controlling the position of HAZsoftening and obtaining the appropriate electric current range, using atwo-step energization in which steel sheets are heated throughpreliminary energization with an early-stage of energization being lowelectric current, and then, a nugget is expanded with a short-time highelectric current.

Investigations were made on a hot-stamped member of an 1800 MPa classhaving a thickness of 1.6 mm under welding conditions shown in Table 1.Condition (1) corresponds to a short-time single energization condition,(2) corresponds to a conventional single energization condition, and (3)corresponds to a two-step energization condition.

As can be understood from the behavior of nugget formation illustratedin FIG. 14 and the distribution of hardness in the spot-welded portionillustrated in FIG. 15, with the two-step energization, it is possibleto obtain an appropriate electric current range equivalent to that withthe conventional single energization and bring the softest zone in HAZclose to the nugget end portion.

TABLE 1 First energization Second energization Compression force TimeElectric current Time Electric current Retention time Electrode (kN)(cyc) (kA) (cyc) (kA) (cyc) Condition (1) Cu - 1% Cr 3.92 9 3.0-8.5 — —10 Condition (2) Dome type 20 3.0-8.5 — — Condition (3) Diameter of top11 4.0 9 3.0-8.5 end: 6 mm

The welding electric current was adjusted so that the nugget diameter of4√t (mm) can be obtained. As for the subsequent energization condition,a condition effective in improving the peel strength, in other words, acondition with which the end portion area of a nugget can be softenedwas selected.

By using a steel sheet of a 980 MPa class, hot-stamped members of a 1500MPa class, and hot-stamped members of an 1800 MPa class, each of whichhas a thickness in a range of 1.6 mm to 2.0 mm, the cross-tensionstrength and the L-shape tension strength were investigated. Table 2shows welding conditions used. I₀ represents an electric current valuein the main energization process.

TABLE 2 Preliminary Subsequent energization energization process Mainenergization process Compression Electric process Suspension ElectricRetaining force Time current Time time Time current time ConditionElectrode (kN) (cyc) (kA) (cyc) (cyc) (cyc) (kA) (cyc) A Cu - 1% Cr 3.92— — 10t + 4 — — 10 B Dome type — — 10t + 4 60 20-50 0.6I₀-0.8I₀ A1Diameter of — — 5t — — A2 top end: 6 5t + 3 0.5I₀-0.7I₀ 5t — — B1 mm — —5t 60 20-50 0.6I₀-0.8I₀ B2 5t + 3 0.5I₀-0.7I₀ 5t 60 20-50 0.6I₀-0.8I₀

Condition a corresponds to a conventional single energization condition,and condition b corresponds to a conventional subsequent energizationcondition. Condition A1 corresponds to a short-time single energizationcondition, and condition A2 corresponds to a two-step energizationcondition. As for conditions B1 and B2, subsequent energization wasperformed for the conditions A1 and A2, respectively. In each of thewelding conditions, welding electric current was adjusted so that thenugget diameter of 4√t can be obtained.

As for the joint strength, measurement was performed according tocross-tension test based on JIS Z3137 (1999) in the case of a crossjoint, and measurement was performed with a test illustrated by aschematic diagram of FIG. 21 in the case of an L-shaped joint.

More specifically, in L-shaped tensile testing, bent portions of twotest pieces each formed by bending a steel sheet into an L shape wereoverlapped with each other as illustrated in FIG. 21, and were joined byforming a spot-welded portion 10 having a nugget 12 formed in theoverlapped portion through resistance spot welding; then, the overlappedportion was pulled in a direction of peeling; and a strength of thespot-welded portion 10 until fracture was measured as the jointstrength.

First, by using a DP steel sheet of a 980 MPa class and hot-stampedmembers of a 1500 MPa class and an 1800 MPa class, each of which has athickness in a range of 1.6 mm to 2.0 mm, the peel strength ofoverlap-welded members and fracture mode thereof were investigated.Table 4 shows welding conditions. t is the thickness of the steel sheet,and I₀ was adjusted so that the nugget diameter 4√t (mm) can be obtainedin each of the sheet sets. Note that the distance D is a distance fromthe nugget end portion area to the softest zone in HAZ.

Table 3 shows effects of improvement of joint strength under theconditions A1 and A2. Table 3 is a table explaining Example related tothe single energization.

TABLE 3 Distance Joint Thickness t t^(0.2) Welding D strength FractureSteel type Joint shape (mm) (mm) condition (mm) (kN) mode Note  980 MPaCross 2.0 1.15 a 1.5 9.8 Partial plug Comparative fracture Example A11.0 12.5 Partial plug Example of present fracture invention 1500 MPa 1.61.10 a 1.3 5.6 Partial plug Comparative fracture Example A1 0.8 8.2 Plugfracture Example of present invention A2 1.0 8.3 Plug fracture Exampleof present invention 2.0 1.15 a 1.5 7.2 Interface Comparative fractureExample A1 1.0 8.6 Partial plug Example of present fracture invention1800 MPa 1.6 1.10 a 1.3 3.2 Partial plug Comparative fracture Example A21.0 3.9 Partial plug Example of present fracture invention 2.0 1.15 a1.5 4.8 Interface Comparative fracture Example A1 1.0 5.5 InterfaceExample of present fracture invention 1800 MPa L shape 1.8 1.12 a 1.31.2 Interface Comparative fracture Example A1 0.8 1.5 Interface Exampleof present fracture invention

As shown in Table 3, with any steel type and any thickness, thespot-welded portions formed through the short-time energization with thecondition A1 and the preliminary energization and the short-timeenergization with the A2 improve the joint strength, as compared withthe conventional single energization with the condition a. It can beconsidered that this is because of the effect in which the stressconcentration on the nugget end portion is alleviated by bringing theposition of HAZ softening closer to the nugget end portion.

Furthermore, as for the L-shaped tensile strength concerning thehot-stamped member of an 1800 MPa class, by applying the short-timeenergization with the condition A1, the joint strength improves byapproximately 25%, as compared with the case of the condition a.

Furthermore, by using the DP steel sheet of a 980 MPa class, andhot-stamped members of a 1500 MPa class and an 1800 MPa class, each ofwhich has a thickness in a range of 1.6 mm to 2.0 mm, the peel strengthof overlap-welded members and fracture mode thereof were investigated.Table 4 shows welding conditions. t is the thickness of the steel sheet,and I₀ was adjusted so that the nugget diameter 4√t (mm) can be obtainedin each of the sheet sets. Note that the distance D is a distance fromthe nugget end portion area to the softest zone in HAZ.

Table 4 shows effects of improvement of joint strength under theconditions B1 and B2. Table 4 is a table explaining Example in the casewhere the subsequent energization is applied.

TABLE 4 Distance Joint Thickness t t^(0.2) Welding D strength FractureSteel type Joint shape (mm) (mm) condition (mm) (kN) mode Note  980 MPaCross 2.0 1.15 b 1.5 15.9 Plug fracture Comparative Example B1 1.0 18.1Plug fracture Example of present invention 1500 MPa 2.0 1.15 b 1.5 14.5Partial plug Comparative fracture Example B1 1.0 18.5 Plug fractureExample of present invention 1800 MPa 1.6 1.10 b 1.5 5.7 Partial plugComparative fracture Example B1 1.0 8.7 Plug fracture Example of presentinvention B2 1.0 8.7 Plug fracture Example of present invention 2.0 1.15b 1.5 5.2 Interface Comparative fracture Example B1 1.0 7.3 Partial plugExample of present fracture invention 1800 MPa L shape 1.8 1.12 b 1.52.2 Interface Comparative fracture Example B1 1.0 4.7 Plug fractureExample of present invention

As shown in Table 4, with any steel type and any thickness, the jointstrength improves, as compared with the conventional single energizationcondition with the condition a. It can be considered that this is notonly because the nugget end portion is tempered and the toughnessimproves as with the conventional subsequent energization technique, butalso because of the effect in which the distribution of hardnessappropriate for alleviating the stress concentration on the nugget endportion can be obtained.

Furthermore, as for the L-shaped tensile strength concerning thehot-stamped member of an 1800 MPa class, by applying the short-timeenergization and the subsequent energization with the condition B1, thefracture mode changes from the interface fracture to the plug fracture,and the joint strength improves by approximately 114%, as compared withthe case of the condition b.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to improve the peelstrength of a spot-welded portion of an overlap-welded member in whichplural steel sheet members, at least one of which contains martensite,are joined at an overlapped portion, and the overlapped portion isjoined at the spot-welded portion. Therefore, the present invention isindustrially applicable.

BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS

-   10 spot-welded portion-   12 nugget-   12B nugget end portion area-   12C meeting portion-   12E nugget end-   14 HAZ-   14H HAZ hardened portion-   14T HAZ softening zone-   14L the softest zone in HAZ

1. An overlap-welded member in which an overlapped portion including aplurality of steel sheet members is joined at a spot-welded portion,wherein at least one of the plurality of steel sheet members containsmartensite; and the spot-welded portion includes: a nugget formedthrough spot welding; a heat-affected zone formed in the vicinity of thenugget; a softest zone having the lowest Vickers hardness in theheat-affected zone; and a tempered area formed between a central portionof the nugget and the softest zone and made out of tempered martensitehaving Vickers hardness of not more than 120% in a case where Vickershardness of the softest zone is 100%.
 2. The overlap-welded memberaccording to claim 1, wherein the plurality of steel sheet membersinclude a hot-stamped member.
 3. An automobile part including theoverlap-welded member according to claim 1.